1. Field of the Invention
The present invention relates to temperature and concentration measurement, and more particularly relates to non-contact temperature and concentration measurement on liquid surfaces using reflectance techniques.
2. Brief Description of the Prior Art
It is desirable to measure temperature at a liquid surface for a number of applications. For multi-component liquids, it is also desirable to measure concentration. Although the surface temperature and concentration are important parameters, it has proven difficult to measure them with conventional techniques.
Some prior measurement techniques rely on direct-contact; i.e., a sensor or probe must come in contact with the liquid to be measured. Such measurement techniques, however, have the drawbacks of the potential for contamination; the inconvenience of introducing a probe into the liquid to be measured; and the invasive nature of the measurement which may influence the process being measured.
Accordingly, there has been interest in non-contact measurement techniques. One well-known prior method is use of a commercial infrared thermometer. Such thermometers exhibit poor accuracy for measurement of temperature of surfaces where the emissivity is not close to unity, for example, liquids and highly reflective metals.
In an effort to overcome the problems associated with infrared surface temperature measurement, efforts have been made to develop laser-based techniques. Such techniques have been disclosed in the following publications: T. Q. Qiu et al., "Novel Technique for Noncontact and Microscale Temperature Measurements," Experimental Heat Transfer, v.6, 231-41 (1993); A. S. Lee et al., "Temperature Measurement by Thermoreflectance at Near Grazing Angles," Proceedings of the 1996 A.S.M.E. International Mechanical Engineering Congress and Exposition, v.59, 77-82 (Atlanta, Ga., Nov. 17-22, 1996); and A. S. Lee and P. M. Norris, "A New Optical Method for Measuring Surface Temperature and Large Incident Probe Angles," Rev. Sci. Instrum., v.68, 1307-11 (1997). However, these prior techniques have focused almost exclusively on temperature measurement of surfaces of solid-state materials; relatively little attention has been paid to liquids.
Optical techniques for measuring concentration in liquids have been known previously. One paper discloses an optical probe which relies on changes in the index of refraction to measure liquid concentration. T. L. Bergman, "Miniature Fiber-Optic Refractometer for Measurement of Salinity in Double-Diffusive Thermohaline Systems," Rev. Sci. Instrum., v.56, 291-96 (1985). However, the techniques set forth in this paper still require immersion of a probe in the liquid. Measurement of salt concentration based on the image distortion of a fine wire as it passes through a variable-concentration liquid has also been disclosed. T. L. Bergman, "Measurement of Salinity Distributions in Salt-Stratified, Double-Diffusive Systems by Optical Deflectometry," Rev. Sci. Instrum., v.57, 2538-42 (1986). This technique measures concentration in the bulk liquid as an integrated effect of concentration variation along the light path. Again, however, immersion of a sensor, in this case, the fine wire, was required with the attendant disadvantages.
In view of the foregoing deficiencies with current techniques for measurement of temperature and concentration at a liquid surface, it would be desirable to develop an apparatus and method for non-contact temperature and concentration measurement on liquid surfaces. In addition to affording a non-invasive measurement technique, it would be desirable if the apparatus and method permitted remote monitoring and location of test and analysis equipment, imperviousness to harsh and corrosive environments, and very high spatial precision. Yet further, it would also be desirable if the apparatus and method were capable of fast response times and high reliability and repeatability.